Recovery of oil from subterranean formations utilizing the natural forces present within the reservoir, or primary recovery, result in a significant portion of the oil remaining in the formation. A variety of supplemental techniques have been employed to recover the oil which remains within the formation after the primary recovery. The most widely utilized supplemental technique is water flooding. Water flooding is accomplished by injection of water, or brine, into the reservoir via injection wells to displace residual oils toward recovery wells. Water alone is not particularly effective at displacing oil because of the high interfacial tension between the oil and the water. The water therefore traps a significant portion of the residual oil in the formation by capillary forces.
Surfactants can be added to water flood injection waters to lower the interfacial tension between the oil and the water, significantly improving oil recovery. Unfortunately, the surfactants are generally not effective in the presence of divalent cations such as magnesium and calcium. Divalent cations typically cause these surfactants to precipitate. Further, the formation rock will tend to absorb surfactant, resulting in a large amount of surfactants being required in order to maintain an effective level of surfactants at the oil-water interface.
Surfactants which are effective in spite of the presence of multivalent cations have been developed, and are taught in U.S. Pat. Nos. 4,446,079 and 4,293,428. These patents disclose propoxylated ethoxylated surfactants which can be designed to result in a low interfacial surface tension in the presence of water phase salinity. The design of the surfactant is altered by adjusting the carbon number of the alkyl group, the number of propylene oxide units included, and the number of ethylene oxide units included. Alteration of those parameters varies the hydrophobe/hydrophile ratio to match the ratio which minimizes interfacial tension between water of the salinity required and the reservoir oil. The examples of U.S. Pat. No. 4,446,079 indicate that surface tensions in the mdynes/cm range can be achieved in 8 percent salinity brine with only 0.01 percent by weight of surfactant. However, the propoxylated-ethoxylated surfactants are relatively expensive. Further, tailoring such a surfactant for specific applications assures that volume production of the surfactants, and the resulting economies of scale will never be achieved. It would be desirable to achieve high levels of oil recovery with less expensive surfactant that do not need to be tailored to best recover oil from specific reservoirs.
It is therefore an object of this invention to provide a surfactant composition and oil recovery method utilizing this composition which, in concentrations of less than 2 weight percent, can recover 50 percent or more of residual oil from clay containing sandpack tube tests. It is another object to provide such a surfactant composition and method which can be utilized in the presence of divalent metal cations. It is another object to provide such a surfactant composition and method which is useful in surfactant flooding of residual oil from subterranean formations.